Misjudging a child's size can cause problems — for instance, mothers of normal weight or even overweight children might encourage their sons and daughters to eat more to gain weight, when in reality, the children do not need to. Excess weight gain before age 5 increases achild's risk for obesityin adolescence, and health problems later in life, the researchers said.

Parents may misperceive their child's weight because they believe a little extra baby fat on their child is a good sign.

"There's this misperception that a chubby infant or toddler is a healthy infant or toddler," said study researcher Erin R. Hager, of the University of Maryland School of Medicine's Department of Pediatrics, Growth and Nutrition. In addition, with so many overweight and obese kids in the United States, the view of what is a normal may be shifting, and now larger is the new norm, Hager said.

The findings suggest parents should be made more aware of what constitutes a healthy weight during toddlerhood, the researchers said.

Overweight toddlers

The study involved 281 mothers from low-income households who had children between ages 12 and 32 months. Mothers were shown seven silhouettes of toddlers of various sizes, and asked to choose the silhouette that best matched their child.

About 30 percent of children were considered overweight by the researchers, based on a ratio of the child's weight and length.

About 70 percent of all mothers in the study were inaccurate in their assessments of their child's size, meaning they chose a silhouette that was at least two sizes larger or smaller than their child's true size.

Mothers of underweight children often knew their child was not healthy—they were 9.5 times more likely to choose the silhouette that matched their child's body size compared with mothers of healthy-weight children.

About 70 percent of mothers of healthy-weight children, and 80 percent of mothers of overweight children said they were satisfied with their child's body size. Four percent of mothers of overweight children even wished their child was larger, the researchers said.

Because mothers in the study were primarily from low-income households, and most were overweight or obese themselves, the findings may not be generalizable to the population as a whole, the researchers said.

Important role for doctors

The only real way for parents to know if their child is overweight is to plot their weight and length on a growth chart for their age, Hager said. Children are considered overweight if they fall in the 85th to 94th percentiles of the growth charts, and obese if they are in the 95th percentile or higher.

When doctors use the charts, they tend to plot weight and height separately, and without both pieces of information, parents end up not knowing that their child is above normal size for his age, Hager said.

A recent study found more than 75 percent of parents of overweight children said they had never been told by their doctor that their child was overweight.